The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent that it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Selective catalytic reduction (SCR) catalysts and diesel oxidation catalysts (DOC) are commonly used with diesel engines for emissions reduction. In the SCR process, nitrogen oxide (NOx) reacts with a reductant which is injected by a dosing system into the exhaust gas stream to be absorbed onto an SCR catalyst. The injected dosing agent (e.g. urea) breaks down to form ammonia (NH3). NH3 is the reductant that is utilized to react with NOx to reduce NOx into nitrogen (N2) and water (H2O).
The SCR process typically requires relatively high exhaust gas temperatures (such as temperatures greater than 200° C.). Traditional diesel engines, however, generate an exhaust gas of low temperatures, generally below 200° C., at cold start and under low load operating conditions. During a cold start phase of a FTP-75 cycle (the Bag I or Phase I of Federal Test Procedures for emission testing), even with a preliminary warm-up strategy, the highest temperature of the SCR unit may still be lower than the temperature required to make the SCR unit effective in reducing NOx emission.